Conventionally, there has been proposed an antenna device in which two antennas are connected to a switching device, respectively, and one of two signals of the two antennas can be input to a receiver through selection by the switching device.
This antenna device can change a radiation pattern by selecting the antennas, providing a diversity effect. However, providing the diversity effect involves a plurality of antennas and receivers, resulting in an increase in a mounting area and a higher mounting cost.
Also, according to another conventional art, a known antenna device has a loop antenna both ends of which are short-circuited each other so as to change a radiation pattern. In this case, a single antenna can allow the pattern to be changed. However, since each terminal of the antenna is short-circuited, there arises a problem that a signal cannot be input or output when a transceiver to process a differential signal is connected to the antenna device. It is noted that because a differential signal, generally, can drastically reduce performance degradation caused from external noises and hard-wiring connected to a radio device, it is said that the differential signal can present a considerably workable improvement in performance of a radio device configured to process a high frequency.
As described above, there have conventionally been problems, such as an increase in the mounting area, a higher cost or a lack of connection capability to a differential transceiver.